1. Field of the Invention
This invention relates to a combiner for a radio receiver which receives signals travelling by different signal paths of a digital transmitter such as a mobile transmitter in which the individual received signals are demodulated after standard frequency conversion.
2. Description of the Related Art
In radio transmission, particularly mobile broadcasting, transmission can be disturbed by multi-path reception. Such multi-path reception generally causes a frequency dependency of the transmission factor which appears as selective fading.
In narrow-band systems, which are systems having a relatively low transmission band width, the transmission factor of a given link is nearly constant within the transmission band width. Changing propagation conditions merely causes level fluctuations of the received signal, and for this reason, signals from different antennas can be superimposed to obtain additional gain without problems. Such superimposition or addition usually is performed in the IF stage whereby a control of the phases of the individual demodulated carriers relative to each other assures that the individual IF signals are coincidence with each other. Terms used for such system are "equal gain" or "maximal ratio combining", which refers to a potential evaluation of the individual signals.
Such a quasi, linear diversity combiner is also generally used in broadband systems. However, in such systems, respective transmission factor is not constant within the transmission band and for this reason, changing propagation conditions also cause variations in the signal distortions which are present. When signals which are differently distorted are superimposed, the individual signals can only be brought into incomplete coincidence despite control of the phases of the demodulation carriers and for this reason, the maximum possible diversity power gain cannot be obtained.
In the method of quasi linear superimposition, a certain coincidence of the signals which are to be superimposed must be produced. The criteria required is usually derived from a comparison of the incoming signals in a continuous manner and this does not represent a problem which cannot be solved when continuous transmission occurs. If discontinuous transmission occurs, as, for example, in the future, Pan-uropean mobile radio network, such a superimposition and combining is not possible without additional considerations. In such a case, successive pulse groups or bursts may have completely different distortion and for this reason, every pulse group or burst must be individually processed. Thus, a criteria for the control procedure in a pulse system is not available.
"Mobile Communications Engineering"by W. C. Y. Lee, 1982, McGraw Hill, pages 291 ff, discloses such prior art "diversity combiners".